1. Field of the Invention
The present invention relates to methods for guiding radiation treatments of animals, especially human patients. The present invention is particularly related to methods for guiding the distribution of radiation in medical radiotherapy processes which generate spherical doses of radiation or, more properly, approximately spherical distributions of radiation.
2. Background of the Art
Tumors of the brain are regarded as special candidates for precision conformal radiotherapy. Historically the term "radiosurgery," introduced by Leksell (Leksell, L. 1951, "The Stereotactic Method and Radiosurgery of the Brain," Acta Chir Scand 102:316-319), was used to describe the method by which many narrow radiation beams were used to irradiate intracranial structures from many angels as an alternative to surgery. The Leksell 201 source.sup.60 Co Gamma Knife.RTM..sup..dagger. used for such radiosurgery, is shown in FIG. 1 (and as with OUR Scientific International, Inc., New York, N.Y., "Rotating Gamma System"). These 201 sources are contained in an upper hemispherical shield. All sources are focused to a single point at a source-to-focus distance of 403 mm. The central beam is at an angle of 55.degree. to the horizontal plane. The sources lie in an arc of .+-.48 about this central ray in the long axis of the treatment unit and at an arc .+-.80.degree. along the transverse axis to the patient couch. The final precise collimation is achieved by selection of one of four interchangeable collimator helmets with 201 channels aligning with the sources that produce 4, 8, 14, or 18 mm diameter fields at the focus (Wu, A. 1992, "Physics and Dosimetry of the Gamma Knife," Neurosurgery Clinics of North America, Vol. 3(1):35-50). A stereotactic frame is attached to the patient under local anesthesia and then the frame is positioned into the collimating channel so that the tumor to be treated is at the focal point of the beams.
Treatment planning optimization for radiation therapy, including, but not limited to gamma knife radiosurgery, linear accelerator treatment (either modified or specifically designed for radiosurgery), Brachy treatment (where radiation dose distributions are supplied by small sealed radiation sources, called seeds), and like processes which seek to distribute radiation over bulk (volumetric versus surface area) portions of a patient, as in the treatment of tumors, benign growths, malignant growths, etc., is aimed at maximizing the dose to the target volume while minimizing the dose to adjacent normal tissues. Since radiation deposits energy all along its pathway, treatment planning optimization is a constrained optimization problem, i.e., primarily comprised of two competing limitations on the dose: 1) maximization for the targets, and 2) minimization for the normal tissues. The index of the dose given to a particular area as compared to the target dose which has been determined or established is referred to as homogeneity. In the target areas, the homogeneity is desirably between 100 to 50%. The area around the target area is the penumbra or shadow region of the exposure, and receives a dose measured as homogeneity usually in the range of 50% to 30% of the 100% of the target dose. The percent dose is measured relative to the dose at the focus of the radiation equipment. Percent dose is relative to dose at the focus. The target dose and the dose at the focus are preselected by the operator according to the specific radiation therapy intended by the equipment.
It has been observed that multiple shot treatment may deteriorate both penumbra and homogeneity compared with single shot treatment. This is due to overlap of adjacent shots and the less than optimal shot weights. Pla et al. (Pla, C. Podgorsak, E. B., Pla, M., Souhami, L., Clark, B. G., and Carson, L. J., 1995, "Considerations on the use of Multiple Isocenters in Stereotactic Radiosurgery," Med Phys, Vol. 22, No. 5:765) recently discussed the problem but did not provide any useful solution. Mathematically, the optimization process can be represented as ##EQU1##
where N is the number of shots being used, S.sub.i is the ith shot, and d and x are the shot size and position, respectively. Thus, the number of shots and the position and size of each shot have to be included as parameters for the optimization, and must be optimized simultaneously. Clearly, the irregularity and size of a target volume greatly influence the number of shots needed and the sized of the shots being used to optimize the treatment.